The present invention relates to microactuators having macroscopic force and displacement. More particularly the invention relates to microactuators comprising a 3-D array of small actuator cells, which are formed from stacked polymer sheets with electrodes.
Most microactuator arrays, used as MEMS (Micro-Electro-Mechanical-Systems) devices, are fabricated in silicon. Despite the many favorable attributes of silicon, it is not always a suitable or ideal material for every application of MEMS. Silicon is brittle and subject to breaking, particularly as the total device size increases. This brittleness limits devices, especially actuators, to relatively small sizes capable of only small displacements and forces. The shapes that can be realized in silicon are typically restricted by crystalline planes or 2-D fabrication processes, and more complicated structures often result in prohibitively high cost and low yield. It would be of great advantage to the art if another material, other than silicon, could be used for MEMS and actuators.
The present invention overcomes many of the disadvantages of the prior art by providing a microactuator that is formed primarily from polymer materials, and further includes an integrally formed package. The actuator array may be similar to that described in U.S. Pat. No. 6,184,608, issued Feb. 6, 2001, to Cabuz et al., entitled xe2x80x9cPOLYMER MICROACTUATOR ARRAY WITH MACROSCOPIC FORCE AND DISPLACEMENTxe2x80x9d, which is incorporated herein by reference.
Cabuz et al. discloses a polymer based microactuator array that is formed from a plurality of generally parallel thin flexible polymer sheets bonded together in a predetermined pattern to form an array of unit cells on the various layers. Thin layers of conductive films and dielectric materials are deposited on the polymer sheets to form a plurality of electrodes associated with the array of unit cells in a conventional manner. Contact is made between layers through flexible plastic interconnect straps as in a conventional flex-tape connector. These interconnection straps can be metalized plastic or conductive polymer, and may be cut from the same polymer sheet as in the stack layers. Layers can be electrically tied together or individually addressed, depending on the degree of control and sophistication of the end use, noting that individual addressing requires more connections so it would involve higher cost but finer control when needed.
The top of the microactuator stack of Cabuz et al. is secured to a top housing and the bottom of the microactuator stack is secured to a bottom housing. The top and/or bottom housings may include an external connector and control circuitry for controlling the microactuator. The top and/or bottom housing may further include one or more levels of metal interconnects, as in a printed circuit board, to route the inputs of the external connector through the control circuitry, and to the sheets of the actuator using one or more of the flexible interconnect straps.
Electrostatic actuators are often susceptible to degradation from various environmental conditions. Corrosion, dust, humidity and even air currents can reduce the reliability and/or performance of the actuator. To protect the microactuator stack from certain environmental conditions, Cabuz et al. provides a flexible protective film around the microactuator stack. This ends of the protective film are attached to the top and bottom housings, respectively. A limitation of the protective film of Cabuz et al. is that it can be difficult to attach the ends of the protective film to the top and bottom housings. Another limitation of Cabuz et al. is that connecting the flexible interconnect straps that are used to interconnect the various polymer sheets of the stack to one another and to the housing can be difficult.
The present invention overcomes many of the limitations of Cabuz et al. by providing a microactuator array with an integrally formed package. The integrally formed package provides protection to the microactuator stack, while requiring fewer assembly steps than the protective film approach of Cabuz et al. In addition, the integrally formed package of the present invention may provide all the necessary interconnections and leads for addressing and distributing power to the various layers in the microactuator stack.
In a first illustrative embodiment of the present invention, a plurality of generally parallel thin flexible polymer sheets are bonded together in a predetermined pattern to form an array of unit cells. A protective package is provided around the microactuator stack, and is mechanically coupled to selected sheets in the stack using one or more bridges.
The protective package may be formed at the same time as the sheets of the microactuator stack. That is, and in one illustrative embodiment, each sheet of the microactuator stack includes an electrode region and a frame region, with the frame region extending around and spaced from the outer perimeter of the electrode region. When stacked, the frame regions form the protective package around the microactuator stack.
To connect the frame region to the electrode region, one or more bridges are provided on selected sheet. The bridges preferably extend between the electrode region and the frame region to form a mechanical connection therebetween. In a preferred embodiment, the frame region, the bridge regions, and the electrode region are cut from the same polymer sheet. During assembly, the electrode regions are bonded together in a predetermined pattern to form an array of unit cells. At the same time, the frame regions are bonded together to form a protective package around the microactuator stack. As such, the protective package may be formed at the same time, and integrally with, the microactuator stack.
The frame region may include one or more contacts and/or vias for providing an electrical connection between adjacent polymer sheets. Conducting traces may also be provided for electrically connecting the contacts and/or vias to selected electrodes in the microactuator stack. The conducting traces preferably are located in the frame region, and extend across one or more of the bridges to a desired electrode in the microactuator stack. By properly positioning the contacts, vias and conducting traces, the stacked frame regions can provide all the necessary interconnections and leads required for addressing and distributing power to any of the layers in the microactuator stack.
The top of the microactuator stack and the top of the protective package are preferably secured to a top housing, and the bottom of the microactuator stack and the bottom of the protective package are preferably secured to a bottom housing. Like Cabuz et al., the top and/or bottom housing may include an external connector and control circuitry for controlling the microactuator. The top and/or bottom housing may also include one or more levels of metal interconnects, as in a printed circuit board, to route the inputs of the external connector through the control circuitry, and to selected locations on the protective package. As indicated above, the protective package of the present invention may distribute the control signals to any layer in the microactuator stack.
In one illustrative embodiment, the frame regions are bonded together to form a billows type package configuration that can expand and retract with the movement of the microactuator stack. This can be achieved by, for example, providing a first bonding region along a first major surface (e.g., top surface) of the each frame region, and a second bonding region along a second major surface (e.g., bottom surface) of each frame region.
For odd layers, for example, the first bonding region may be positioned closer to the outer perimeter of the frame region than the inner perimeter, and the second bonding region may be positioned closer to the inner perimeter of the frame region than the outer perimeter. For even layers, the first bonding region may be positioned closer to the inner perimeter of the frame region than the output perimeter, and the second bonding region may be positioned closer to the outer perimeter of the frame region than the inner perimeter. As such, each of the bonding regions may be align with the bonding regions of adjacent sheets, and the frame regions, when stacked and bonded, may form a billows type package configuration that can expand and retract with the movement of the microactuator stack.